1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for forming a thin film.
2. Description of the Related Art
The efficiency with which semiconductor devices are manufactured is a function of each and every process performed during manufacture. In other words, if the time required to complete one manufacturing process is reduced, the overall manufacturing efficiency is improved.
Many of the manufacturing processes performed during the fabrication of a semiconductor device take place in a reaction chamber. The transit time for wafers, or for a sheet of wafers, hereafter referred to as "wafers," to and from the reaction chamber, as well as the preparation time of the reaction chamber itself, all contribute to the processing time. As an example, a conventional batch process used to form a thin film on a number of wafers will now be described.
To begin the process, a batch of wafers is loaded into a wafer cassette. From the wafer cassette, wafers are transferred one after the other into a reaction chamber by a storage elevator. Once a first wafer is in the reaction chamber a thin film is formed, following which the wafer is removed and returned to the wafer cassette via the elevator. After removing the first wafer, the reaction chamber is cleaned. Once the reaction chamber is cleaned, a second wafer is carried from the wafer cassette by the elevator to the reaction chamber. The process thus repeats throughout the stack of wafers in the wafer cassette.
Of note, the conventional thin film forming process requires that the reaction chamber be cleaned following thin film formation on each wafer. However, since as many as 25 wafers (plus one additional test wafer) may be loaded into a wafer cassette for any given thin film process batch, the reaction chamber must be cleaned, using conventional technology, 25 times.
Table 1 shows a conventional cleaning recipe for the reaction chamber when the thin film formed in the reaction chamber is a plasma enhanced oxide film.
TABLE 1 Item 1K 1.5K 2.5K 4K Step 1 2 2 2 2 3 4 Activity Set flow Clean Clean Clean Clean Purge Pump Step control Time Time Time Time Time Time Time factor Time(sec) 5 20 30 40 50 10 20 Pressure (mT) 5 5 5 5 5 open open RF(W) 0 800 800 800 800 0 0 Temperature 0 0 0 0 0 0 400 Space (mils) 600 600 600 600 600 600 600 N.sub.2 O 550 550 550 550 550 0 0 CF.sub.4 1750 1750 1750 1750 1750 N.sub.2 0 0 0 0 0 2000 0
As can be seen from the Table 1, cleaning of the reaction chamber consists of four steps. The first step is a set flow step, the second step is a cleaning step, and the third and fourth steps are purge and pump steps, respectively. The controlling factor for each step is time. The first step takes about 5 seconds, the second step between 20 and 50 seconds, depending on the thickness of the oxide film formed, the third 20 and fourth steps take about 10 and 20 seconds, respectively. With these process steps, the air pressure during the first and second steps remains uniform at about 5 mTorr, but during the third and fourth steps pressure within the reaction chamber is arbitrarily adjusted according to the specific purge or pumping steps.
Further, the induced power (RF) is zero during the first, third and fourth steps, but is 800 Watts during the second step. The temperature is maintained at 0.degree. C. during the first second, and third steps, and increased to 400.degree. C. during the fourth step. Space refers to the distance a susceptor, on which a wafer is loaded, is moved up and down, and this space remains uniform at 600 mils throughout the cleaning process.
The gases induced into the reaction chamber during the cleaning process are N.sub.2 O, CF.sub.4 and N.sub.2. In particular, N.sub.2 O and CF.sub.4 are induced during the first and second steps. N.sub.2 is induced during the third step. N.sub.2 O is induced to 550 sccm (standard cubic cm), CF.sub.4 is induced to 1750 sccm, and N.sub.2 is induced to 2000 sccm.
In Table 1, four possible second steps are specified. The choice between these four possible second steps, labeled 1K, 1.5K, 2.5K and 4K, is made in relation to thickness of the thin film previously formed in the reaction chamber.
As described above, the conventional, four step reaction chamber cleaning process is performed following the formation of a thin film on each wafer, as it is sequentially placed in the reaction chamber from a wafer cassette. Thus, the four cleaning steps are performed a number of times, depending on the number of wafers in the batch. Also, as shown in Table 1, since about 55 to 85 seconds are required to complete the four cleaning steps, about 22.95 to 35.42 minutes are required to clean the reaction chamber during a batch process of 25 wafers loaded in a wafer cassette. Again, this is only the cleaning time for the reaction chamber.
This is a considerable amount of time. A significant reduction in cleaning time would increase overall manufacturing efficiency.